When a gun mounted in a closed cab vehicle is being fired, it is desirable that the residual gasses resulting from the firing of one round be removed from the gun bore to prevent noxious gases from entering and accumulating in the crew compartment of the vehicle. Since it takes a few seconds for the crew to open the breech after the gun is fired and since the gases can only be evacuated from the gun bore when the breech is open, it is necessary for the forces causing the evacuation to be active for several seconds following the firing.
Muzzle mounted devices for purging the bore of residual propellant gases, known as bore evacuators, have been in use in various configurations for a number of decades, see U.S. Pat. No. 2,791,940 of May 14, 1957. In its simplest form it is essentially a closed annular reservoir surrounding a portion of the muzzle and tapped into the bore by a series of angled ports. During the firing cycle, the reservoir is pressurized to some operating pressure with propellant gases through the angled ports. As the pressure in the bore drops below that of the reservoir, the pent up propellant gases begin to discharge back into the bore through the ports which are angled towards the muzzle so as to induce a flow of air through the open breech, thus purging the bore.
In some gun designs, an additional set of ports that are located closer to the breech communicate with the reservoir so as to introduce more gas into it, but in order that the gas only pass from the reservoir to the gun bore via the first set of ports, check valves are provided for the additional ports. Since the additional check valves are in close contact with the high pressure high temperature gases in the bore, they are subjected to severe thermal and mechanical loads. Consequently, the valves are made with special alloys and frequent maintenance is required for proper functioning. Severe erosion and mechanical failure of the check valves are common.